For operation in low or ultra-low power environments it is important to be able to operate from a variable power supply. Examples of low power environments include radio frequency ID (RFID) applications, as well as devices which measure vibrations in a structure. In such devices, it is not uncommon to collect limited and intermittent amounts of energy from an outside source such as, for example, light, vibrations, etc. In an attempt to keep form factor and cost low the devices do not have a typical power supply, e.g., AC adapter, batteries, large capacitors or other supply storage devices. Due to this lack of any typical power supply in these devices, the available power is intermittent as is the supply voltage, and as such, the logic clock frequency must be changed to meet timing.
Control of the load (logic) to efficiently use the voltage supply variation is complex and the process and circuitry used in this complex control consumes energy. To control the voltage and frequency independently requires a processor (or state machine) sequencing that insures all frequency settings can be supported by corresponding voltages. In addition, using this type of control in an environment with inexact tolerances will make inefficient use of available power.
More specifically, in known systems, it is necessary to build a frequency look-up table which includes a listing of frequencies that support respective voltages. However, it is not a trivial task to build such a look-up table since the relationship between voltage and frequency is not a straightforward function; that is, frequency and voltage do not have a linear relationship. To build a look-up table it is thus necessary to perform a complex timing analysis for each circuit at different voltages to determine respective frequencies. This timing analysis can then be used to create frequency look-up tables.
Also, a state machine or processor may be used to determine the required voltage/frequency relationship. However, the use of a state machine or process is very costly in power consumption. This, of course, will decrease the overall performance of the device. Also, the use of a state machine is very complex since it requires a lot of circuitry.
By way of a more specific example, in current systems, in order to minimize power for a given performance power consumption currently two controls are necessary, voltage and clock frequency. This control could be internal or external. Voltage and clock frequency must be controlled carefully to insure that the clock frequency can be supported by any given voltage. The internal or external controls provide control to a DAC and a divider, as shown in FIG. 1. In this example, the logic chip is driven by a programmable power supply. When low power operation is desired (trading off maximum performance) the clock frequency can be reduced (via the oscillator/divider) which, in turn, allows the power supply to be reduced. In such a system, the supply voltage cannot be reduced without first reducing the clock frequency. If the supply voltage is reduced without first reducing the clock frequency, timings will not be met. In such known systems, the oscillator frequency does not track the power supply; instead, control over the power supply and/or oscillator/divider is by the controlled logic and an external logic controller.